Various aspects are of high priority when dealing with the combustion of solid objects or particles e.g. in an industrial power plant and/or a waste incineration plant and/or the like. Such aspects include fast and efficient energy production, waste management, and the desire to minimize pollution as much as possible without sacrificing efficiency. There is also an increase in the political and popular demand for green profiles within the industries of waste disposal and/or energy production.
One main inhibitor in obtaining an efficient combustion process is the presence of ash or the like, which at some point in the combustion process of a solid object or particle typically will be present on the outer surface.
Further, the energy and mass exchange at the surface of the solid(s) to be burnt is largely determined by the character of the flow of the combustion gas surrounding the solid(s) and more specifically by the character or presence of a so-called laminar sub-layer. Heat transport across the laminar sub-layer will typically be by conduction or radiation, due to the nature of the laminar flow while mass transport across the laminar sub-layer typically will be solely by diffusion.
Various methods and systems exists that aim at improving a combustion process.
Patent specification U.S. Pat. No. 4,592,292 relates to a method and apparatus for the combustion of large solid fuels. High particle velocity sound is generated by a resonator and is used to provide a reciprocating movement of combustion air and combustion gas through solid particles on a grate. The sound resonator is located in a chamber together with the grate and yields a standing sound wave with a maximum frequency of 60 Hz, preferably 30 Hz, optimal less than 20 Hz, across a bed of solid fuel.
Patent specification SE 7701764-8 discloses a method of combusting atomized solid, liquid or gaseous fuels. Only atomized fuels and not larger solid objects or particles are addressed. The atomization of the fuel into very small particles is done by disintegrator of various types (for solid fuels) or atomizers (for liquid fuels).
A problem addressed in this specification is that due to the fine atomization of the fuel it is hard to obtain oxidation of the atomized particles since the particles fast obtains the velocity of the combustion air/gas, i.e. no difference between the velocity of the particles and the surrounding air, due to the small mass of the atomized particles. During combustion, each atomized fuel particle will be surrounded by a number of combustion gases (like carbon oxide, etc.) which prevents oxygen to be in contact with the fuel particle, which prolongs the time of combustion and causes a physical extension of the combustion flame.
A proposed solution for overcoming these disadvantages is to supply sound energy to the combustion flame from a sound emitter, so that the velocity of the fuel particles becomes different from the velocity of the air particles due to the different masses of the fuel particles and the air particles.
It is mentioned that the sound energy can be supplied to the flame e.g. using a siren. It is further mentioned that various sound frequencies can be used. Non-audible sound (i.e. infra-sound or ultra-sound) can be used due to sound-environmental considerations, i.e. to reduce noise. Further, it is mentioned that ultra-sound can be used for momentarily heating of the fuel or the fuel/air-mix. It is mentioned that sound energy can loosen ash from the atomized particles but it is also mentioned that this requires addition of an additive or some other means to make the ash porous or loose.
Patent specification JP 01095213 discloses an incinerator for cleaning a waste gas where the incinerator comprises a first and second incinerating chamber. The combustion gas, containing un-burnt parts, is introduced into the second incinerating chamber. Secondary air is provided through rotating air holes, rotating the air, into the second incinerating chamber.
Patent specification U.S. Pat. No. 5,996,808 relates to a method and a process for separation of carbon from fly ash of coal burning plants. An acoustic field is imposed in order to segregate the unburned coal from raw fly ash.
Patent specification U.S. Pat. No. 4,919,807 discloses an ultrasonic vibrator tray for separating particles from ash fragments. A transducer is mounted on the underside of the tray to induce vibrations in the slurry of particulate material.
Patent specification U.S. Pat. No. 5,680,824 discloses a process for burning solids with a sliding fire bar system provided with an airing system to optimize the combustion process.
Patent specification U.S. Pat. No. 5,419,877 discloses an acoustic barrier separator for industrial power plants. A sound wave is used for removal of small particles, such as fly ash, in gas.
Patent specification U.S. Pat. No. 5,785,012 discloses improvement of combustion in a combustion chamber, e.g. in a boiler, using acoustic energy where means for generating acoustic energy is located in the chamber in such a way that the energy is supplied to the chamber whereby particles and gasses is supplied with energy thereby improving the combustion process. The disclosed means for generating the acoustic energy are one or more acoustic horns located above in the combustion chamber. Alternatively, the means may be an electronic acoustic generator e.g. coupled to loudspeakers and amplifiers.
It is disclosed that the horn preferably operates at frequencies selected from 100-500 Hz, i.e. non-ultrasonic sound.
Patent specification EP 0144919 discloses a method of combustion of solids using a low-frequency sound generator.
Patent specification DE 1061021 discloses an apparatus for reducing the size of solids, primarily coal, using ultrasonic sound. The apparatus leads the solid coal through a conduit to the combustion chamber where the coal is reduced in size using reflection surfaces and an ultrasonic generator.
Patent specification DE 876439 discloses amplification of sound waves in a boiler where the amplified sound waves are provided to an acoustic horn via a funnel.
Patent specification EP 1162506 discloses an acoustic soot blower that removes dust at different temperatures depending on a gas pressure.